1930
Chem. Mater. 2005, 17, 1930-1932
Solid-State Electrochemical Micromachining Kai Kamada,*,† Kazuyoshi Izawa,‡ Yuko Tsutsumi,‡ Shuichi Yamashita,‡ Naoya Enomoto,† Junichi Hojo,† and Yasumichi Matsumoto‡ Department of Applied Chemistry, Faculty of Engineering, Kyushu UniVersity, Fukuoka 812-8581, Japan, and Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto UniVersity, Kumamoto 860-8555, Japan ReceiVed February 8, 2005 ReVised Manuscript ReceiVed March 10, 2005
Surface microstructuring of substrates plays a significant role in microelectronics and optoelectronics. The process has recently been extended to the fabrication of chemical systems such as microreactors1 and micropumps.2 The textured surfaces are fabricated using one of two general approaches. One is the topographic deposition of desired materials onto the surface.3 This is the “build-up” type approach, in which the surface is constructed from the molecular or atomic scale. Micromachining belongs to the other category and can be considered to be a “shaving off” type texturing method. Electrochemical micromachining (EM), which works by local dissolution of a conducting substrate (metals, semiconductors) under an applied anodic bias in solution, is one of the most widely used methods because it requires simple equipment and enables more rapid etching than other techniques such as ion beam milling and laser abrasion. However, a liquid electrolyte, which is difficult to handle, is required as a conducting medium between the two electrodes. In addition, high-resolution patterning requires masking. The latter problem is solved by the combined use of position selective laser and ion beam irradiation to draw a minute pattern,4-6 or the application of an ultrashort voltage pulse.7-9 In this communication, we propose a simple, novel route for solid-state micromachining using an anodic electrochemical reaction at the microcontact between the ion conducting microelectrode and metal substrate. More concretely, the metal substrate is locally incorporated into the ion conductor in the form of metal ions via the microcontact * To whom correspondence should be addressed. E-mail: kamada@ cstf.kyushu-u.ac.jp. † Kyushu University. ‡ Kumamoto University.
(1) Wan, Y. S. S.; Chau, J. L. H.; Gavriilidis, A.; Yeung, K. L. Microporous Mesoporous Mater. 2001, 42, 157. (2) Xu, D.; Wang, L.; Ding, G. F.; Zhou, Y.; Yu, A. B.; Cai, B. H. Sens. Actuators 2001, A93, 87. (3) Tan, B.; Venkatakrishnan, K.; Tok, K. G. Appl. Surf. Sci. 2003, 207, 365. (4) Chauvy, P.-F.; Hoffmann, P.; Landolt, D. Electrochem. Solid-State Lett. 2001, 4, C31. (5) Chauvy, P.-F.; Hoffmann, P.; Landolt, D. Appl. Surf. Sci. 2003, 211, 113. (6) Teo, E. J.; Breese, M. B. H.; Tavernier, E. P.; Bettiol, A. A.; Watt, F.; Liu, M. H.; Blackwood, D. J. Appl. Phys. Lett. 2004, 84, 3202. (7) Schuster, R.; Kirchner, V.; Allongue, P.; Ertl, G. Science 2000, 289, 98. (8) Allongue, P.; Jiang, P.; Kirchner, V.; Trimmer, A. L.; Schuster, R. J. Phys. Chem. B 2004, 108, 14434. (9) Kirchner, V.; Xia, X.; Schuster, R. Acc. Chem. Res. 2001, 34, 371.
under a dc bias. The substrate is micromachined as a result of the continuous application of an electric field, which eliminates the need for wet-processing. The present method, which we shall refer to as solid-state electrochemical micromachining (SSEM), can easily control the machining size and depth by adjusting the contact area or other EM parameters. Moreover, scanning the microelectrode under an applied electric field results in a fine-patterned surface. The following sections will focus on the fundamental mechanism behind SSEM and present some experimental results. The model for ion migration in the SSEM system is depicted in Figure 1a. The β′′-Al2O3 (typically, Na-β′′Al2O3), which conducts a variety of metal ions in the interlayer of spinel blocks, was employed as an ion conductor. The solid-state electrochemical cell consists of an Ag plate (cathode)/pyramid-like Na-β′′-Al2O3 microelectrode10/ target metal substrate (thickness: 0.5 mm; M: Ag or Zn; anode) system. To reduce the mechanical stress at the microcontact to a minimum and remain constant, the experimental setup was devised with only the weight of the microelectrode (ca. 0.02 g) to the metal substrate. When a dc electric field is applied to the cell, the local region of the M anode near the solid-solid interface is electrochemically oxidized to Mn+,11 which, in turn, is injected into the Naβ′′-Al2O3 via the microcontact. Na+ migrates through the β′′-Al2O3 and is deposited as Na metal at the Ag cathode/ Na-β′′-Al2O3 interface. Na metal immediately reacts with the O2 and CO2 in the air to form Na2CO3. Thus, the electrolysis mechanism corresponds to the electrochemical substitution of Mn+ for Na+ in the β′′-Al2O3. As a result, the M anode electrochemically dissolves as Mn+ into β′′-Al2O3. Since the contact radius at the Na-β′′-Al2O3/M interface is on the order of 10 µm,12 a position selective dissolution occurs at the microcontact, and thus SSEM is achieved. SSEM was carried out under galvanostatic conditions. Figure 2 shows the time dependence of applied voltage during a typical constant-current electrolysis run without (a) and with (b) tracing the β′′-Al2O3 microelectrode along the surface of the Ag plate. When the microelectrode was fixed on the Ag surface (Figure 2a), the applied voltage gradually decreased except during the initial stage of electrolysis (
